An apparatus and method is provided for generating one packet with transmission data and transmitting the packet from an access network transceiver system (ANTS) to a plurality of access terminals (ATs) in a mobile communication system including the ATs and the ANTS which are capable of performing packet data communication with ATs located in coverage thereof. The method includes the steps of generating a medium access control (MAC) header including information on a receiving AT's address, a length and format for transmission data, generating a MAC payload, by consecutively connecting data units to be transmitted to the receiving AT, and generating a MAC trailer. The ANTS pads ‘0’ bits to the MAC header if a predetermined MAC size is greater than a sum of lengths of the MAC header, the MAC payload and the MAC trailer.